1. Field of the Invention
This invention relates to a composition and method of manufacture of an aqueous wax emulsion and its use in the production of water-resistant gypsum products such as water-resistant gypsum board or panels
2. Description of the Prior Art
The production of gypsum based materials involves a mixture of calcium sulphate hemihydrate with water. The mixture is allowed to hydrate or set to the much harder calcium sulphate dihydrate and the excess water is removed by drying through conventional methods. Gypsum products such as wallboard panels are produced from a hardened gypsum core sandwiched between a set of paper liners which represent the outside surfaces of the wallboard. To overcome problems of water absorption by the hygroscopic gypsum which then weakens the wallboard, other additives such as asphalt-wax emulsions, wax emulsions and polyvinyl alcohols were incorporated with the gypsum in order to produce a water-resistant wallboard.
Some of the problems associated with the known wax emulsions are as follows:
1. Wax-asphalt emulsions are generally unstable--that is, the emulsion tends to separate over time into a liquid phase and a solid crust floating on top. The crust is not easily re-emulsified with mixing and can form lumps in the mix. When the board is produced, the un-emulsified asphalt component bleeds out of the set gypsum core and shows up as black spots on the paper liners. PA1 2. The wax-asphalt is black because of the asphalt component which makes the gypsum core in the final board product off-colour. PA1 3. Asphalt from crude petroleum refining processes can vary in properties and therefore increase the variability in emulsion usage, end-product quality and wallboard manufacturing. PA1 4. Other wax substitutes, such as montan or lignite, while providing good water repellency, are known to cause a buildup of dark or black sludge in process tanks used for emulsification. PA1 5. Many wax emulsions based on montan or other natural or synthetic ester waxes, while they are more readily re-mixed than asphalt wax emulsions, still tend to separate during storage. This separation or "creaming" effect results in inconsistent performance characteristics. PA1 6. Emulsions based exclusively on fatty acids or other alkyl acids, while providing superior storage properties and water repellency, have an adverse effect on the gypsum slurry. PA1 1. Allows for the use of water-repellent materials in the emulsion product. PA1 2. Allows for the substitution of the wax-asphalt component with a compound which does not produce an off-colour gypsum wallboard. PA1 3. Allows use of substitutes for asphalt with improved consistency of properties and performance. PA1 4. Allows use of substitutes for montan wax with improved consistency of properties and performance PA1 5. Provides an emulsion of improved stability over asphalt-based emulsions. PA1 6. Provides an emulsion of improved stability over existing wax emulsions. PA1 7. Results in process equipment with lower maintenance and cleanup procedures. PA1 (i) blending a hydrocarbon wax and an alkyl acid and/or alkyl ester in a molten state to form a wax blend; PA1 (ii) solubilizing polyvinyl alcohol in water to form an aqueous solution; PA1 (iii) mixing the wax blend and the aqueous solution at a temperature above the melt point of the wax blend to form an emulsion; and PA1 (iv) cooling the emulsion to form a stable wax emulsion.
A wax emulsion formulation is proposed in U.S. Pat. No. 5,437,722 wherein the formulation consists of a paraffinic hydrocarbon, montan wax and polyvinyl alcohol. Although providing some water repellency to the gypsum wallboard there are deficiencies in such formulation due to the crude nature of the montan wax which is a product containing a variety of components. For this reason, it results in discoloration of the resulting emulsion and fouling of the handling equipment. Further, the quality of the emulsion formulations using montan wax is variable since the composition of such component is not consistent.
Therefore, the present invention seeks to provide a wax emulsion which: